ITC's intelliCAD or ODA's Teigha? They are open but to their alliance/members not to common public which left only OpenCASCADE. However OpenCASCADE is more suitable for 3D parametric modeling not general 2D drafting, in fact it looks like an attempt by "the big player" to promote their base foundation (against the mighty Autodesk of course). Why I said that? cause their development in recent version is really lacking of interest and motivation!
AutoCAD LT's clone invasion
To the blatant monopolistic attitude of Autodesk (almost as worst as old-day M$) competitors still not give up to give users "freeware" of 2D drafting package in a hope that they might ditch DWG or -more recently- AutoCAD LT. The release of DoubleCAD XT and DraftSight certainly an appealing offer to me who have separate 2D and 3D package for the workflow. DraftSight from Dassault Systemes, the less nagging (but has annoying yearly online activation) of the two is using QT as their GUI and both use ODA's Teigha for DWG compatibility. I see these kind of lure won't recede anytime soon even after Autodesk lawsuit to ODA in 2006 for DWG infringement. All I can say there is no real interest in open source community whatever the big player do.
What Open Source have?
LibreDWG: (a fork of hackish libDWG) which support DWG R15 (I have tested it and it's good), if ever this not developed further it's more or less reasonable
QT: especially GLC-lib which demonstrate fast OpenGL implementation or even the forthcoming Quick3D
Cairo? not sure unless it has maturer OpenGL backend
OpenCASCADE: like I said earlier...
libjpeg, librsvg, libpng, libtiff & libwmf: the graphics format standard
missing: 2D Constraint Solver? that give on-screen drawing aids
FreeCAD: strong 3D tendency
QCAD (is it still developed?): only good if you never known AutoCAD before, I have installer for Windows here
Archimedes: lack of feature, seems unclear where it head to
etc... there even java based CAD!
The Pro always said: in the end you need to extend them to suit your custom needs. This is exactly how Maya with it's MEL+Python or AutoCAD with it's AutoLISP+ARX really shine despite the new approach/technology introduced by competitors. Users' accumulated custom solutions seem not meant to be compromised. Then could it be the next open source CAD based on PySide? It is lame to whine but I want to say please don't forget to implement API for scripting...
Currently I have no OSS in my architectural CAD workflow (embarrassing eh?)
Design Review: collaboration with 3rd-party (in DWF only)
TrueView: "proper" viewing and DWG conversion
SketchUp 7.1 with plugins: 3D modeling, not even considering Blender
VE-Ware: analysis, will reconsider Energy+ though
Kerkythea 2008: 3D rendering, will reconsider LuxRender 0.8 with OpenCL acceleration
DraftSight+DoubleCAD XT: 2D drafting (nothing wrong with DraftSight I just need the ability to open skp file)
Free as in beer! oouch